The question now is whether the whir of a computer hard drive heralds yet another sweeping revolution - a series of advances that converge to alter the whole shape of the economy - or whether the productivity gains in the past few years will prove a flash in the pan.

Growth in productivity, or worker output per hour, has been the magic wand that has helped keep inflation at bay even as the United States glides through a 10th year of its record expansion and the jobless rate lingers at a 30-year low.

The increase in worker output, which allows companies to churn out more goods and services without adding to their payrolls, also drove the longest stocks bull run in history, although earnings jitters lately have taken a toll.

The term "New Economy" that has dominated business-page headlines can be misleading as it implies a distinct sector that can be contrasted with an "Old Economy" sector, economic historian Paul David said. Instead, digital computers and high-speed electronic networks have melded to create a "general-purpose" technology whose effects are spreading throughout the economy.

Mr David, a professor at Stanford University and at Oxford,said the high-tech revolution is akin to electricity in the way it revolutionised industry, and had a broad economic impact.

Electricity was first used only for lighting but eventually, when individual electric motors could be put on machines and tools, the whole architecture of factories was transformed in a way that created huge cost savings.The individual motor let factory production migrate from multistory buildings to one-floor operations, which made possible the fast and cost-effective assembly line layout pioneered by automobile titan Mr Henry Ford and copied by innumerable manufacturers.

Can Internet measure up?Yet there are sceptics out there. Perhaps the best known is economist Mr Robert Gordon of Northwestern University, who puts productivity gains down to cyclical factors and says the Internet cannot hold a candle to the lightbulb or the flush toilet for improving the human condition.

"Outside of the 12 per cent of the economy engaged in manufacturing durable goods, the New Economy's effects on productivity growth are surprisingly absent," he wrote in a May 2000 draft of a paper on the New Economy.But a growing number of thinkers believe the information technology onslaught - with all the inventions that spring from and alongside the microprocessor, from the Internet to the genome project - does qualify for revolutionary status.

Mr David said the much faster rate of labour productivity gains in service sectors cannot be put down simply to cyclical demand growth, particularly this late in the expansion.

"I believe that we are in the midst of a great revolutionin technology, one that we can see only in the wake of new capital that this has spawned," Federal Reserve Bank of Cleveland, president, Mr Jerry Jordan trumpeted to the Ohio Aerospace Institute earlier this month.

In the first industrial revolution powered by the steam engine, Mr Jordan said, productivity growth tripled from pre-revolution levels. "Instead of doubling every 175 years the nation entered an area where citizens could expect to see living standards double every 60 years."With electricity came the second industrial revolution and living standards began to double every 25 years.

Now the United States is poised to reap the rewards of the latest revolution, he added.

But when does it all end?If economists could pinpoint where on the technological curve the United States sits now, they could predict when the productivity advances will level off. But they cannot.

There are rough parallels between now and the 1920s - a productivity slowdown followed by a sharp upward spike, government budget surpluses, a general rise in work force skills and a buoyant economy that helped technology spread.

History no crystal ball But while the similarities can yield insights, economists say it is impossible to use history as a crystal ball.When a key new invention leaves the drawing board, there is a fairly predictable pattern for how its impact spreads through the economy. For example, there were fewer than two telephones per 100 people in 1900, but that rose five-fold to 10 by 1916. Eventually growth leveled off as they became a ubiquitous feature in offices, just as computers are today.

A wave of interrelated innovations like the communications explosion is harder to peg since no one knows what inventions will arise as people find new ways to use a technology, Mr Joel Prakken, chairman of Macroeconomic Advisers, said.

Mr David used the Second Industrial Revolution's patterns to predict correctly that productivity would shake off the doldrums that persisted from the 1970s through the early 1990s, given time for the technology to permeate into the economy.

But he said it is tough to use the past to forecast when gains will level off. "To be able to say how far are we along in this reorganisation as a blanket statement is really very hard. The penetration is really quite uneven - it's gone quite far in some areas and not at all in others," he said.

"There is a point where you begin to reach saturation -where the opportunities begin to become narrower and narrower. We haven't reached that yet. And the potentialities of this technology are not near to being exhausted."

Mr David says new productivity gains will come in three areas now in their infancy. The first is specialised information appliances: industry-specific computer applications that will ease tasks in lower-tech industries. For example, a uniform laundering firm has implanted a chip into each garment and uses scanners to sort faster than was possible with laundry tags.Business-to-business commerce, in which buyers and sellers of goods link up via the Internet, is the second area where the future holds big potential gains.

The third is telecommuting, where workers use mobile communications devices and broadband connections to link to their office networks from remote locations. Companies, some of which are experimenting with partial teleworking, could avoid the cost of maintaining offices in congested urban areas, cut commuting time and tap into labour pools where demand, and thus wages, are not so high.

But even the most optimistic cheerleaders of the technological revolution do not believe it makes the US economy immune to recessions, Macro-economic Advisers' Mr Prakken said. The Great Depression of the 1930s came hard on the heels of nine years of miraculous growth.